1. Field of the Invention
The invention generally relates to a hydrostatic seal for turbine engines. Specifically, the invention is a seal system including primary and secondary seals for use between a shaft and a housing to restrict the flow of a fluid from a higher pressure region to a lower pressure region. A combination of hydrostatic and/or hydrodynamic design elements improves the stiffness of an air film provided by the otherwise hydrostatic seal.
2. Background
A typical gas turbine engine includes numerous seals so as to prevent the recirculation of flow in the compressor and turbine stages, to meter cooling air, and to ensure that the main flow does not enter into the rotor bearing cavities. Such seals typically include stationary ring-shaped structures which operate within close proximity to rotating parts. Several exemplary seals include labyrinth, brush, and air-riding or film-riding systems.
A labyrinth seal is a mechanical device used to prevent the leakage of air or other fluids. A labyrinth seal typically includes a plurality of grooves interlocking with annular runners which restrict leakage by reducing the flow area. The gap or clearance between grooves and runners must allow for bearing clearance, shaft vibrations, deflections, thermal growth, and other similar factors in order to avoid contact between and damage to the grooves and runners. As such, the size of the gap within a labyrinth seal is proportional to the diameter of the seal, typically resulting in more flow leakage and lower efficiency as the size of the seal increases. Also, labyrinth seals by design include significant redundancies, which greatly increase the weight of larger diameter turbines.
A brush seal is another mechanical device used to prevent the leakage of air or other fluids. A brush seal could include a densely packed bed of directionally compliant bristles between a pair of plates. Bristles are oriented so as to contact or nearly contact a rotating part. Brush seals are credited with decreasing leakage within turbine-type applications; however, pressure and temperature considerations limit their applicability in compressor and turbine stages. Also, brush seals experience substantial wear and generate significant heat during the transient operations of a turbine.
An air-riding or film-riding seal is a non-contact device which exploits the pressure difference across the seal to induce a thin film of air in the gap between the face of the seal and a rotating part. The gap is independent of the diameter of the seal, thus avoiding the leakage, wear, and inefficiencies associated with larger labyrinth and brush seals.
Air-riding seals include hydrodynamic and hydrostatic systems. A hydrodynamic seal typically includes spiral grooves and relies on the relative surface velocity between a rotating part and the seal to establish the lift force to form and maintain an air film between the static seal and moving part. A hydrostatic seal relies on the pressure differential between upstream and downstream regions to form and maintain an air film therebetween. Accordingly, hydrodynamic seals are more efficient at higher surface velocities, whereas hydrostatic seals are better suited when pressure differentials of significant magnitude are exploitable.
Gas turbine engines for use with military and commercial aircraft must operate at higher temperatures to achieve the performance and efficiency requirements not required in other applications. The cooling of components within such engines is of the utmost importance.
Typically, critical high temperature components within a turbine engine are cooled by diverting air from the engine; however, this approach lowers component efficiencies and adversely affects inlet temperatures. It, therefore, becomes critical to minimize the amount of air diverted to cool the turbine. Compounding this problem is coolant leakage, which results in both higher amounts of flow being bled off than is required for cooling and a drop in the supply to the engine. Therefore, the ability to provide and maintain sealing throughout a turbine engine is essential in order to function properly.
Current gas turbine engines primarily use labyrinth knife-edge seals to meet this requirement. While these seals have been in use for many years, they have reached their limit in terms of leakage reduction. In addition, it is now generally recognized that their performance deteriorates over time, resulting in ever increasing flow leakage over the life of the seal.
Brush seals have been incorporated into at least one family of gas turbine engines to reduce leakage. In general, brush seals are an improvement over labyrinth seals; however, brush seals degrade with time as bristles are worn because of contact with a rotating part.
Some of the problems described above have been addressed by embodiments of the hydrostatic seal disclosed by Pope in U.S. Pat. No. 6,145,840 entitled Radial Flow Seals for Rotating Shafts which Deliberately Induce Turbulent Flow along the Seal Gap, which is incorporated herein in its entirety by reference thereto.
Pope discloses a face seal for a rotating shaft for use between normally high and normally lower pressure regions. A seal ring is shaped to form a gap between the ring and a runner surface along a shaft. The seal ring includes a ring-shaped sealing surface with a like-shaped seal dam extending therefrom. The gap converges in the direction of fluid flow and creates turbulent flow along the seal. A servo system coupled to the seal ring moves the ring away from the runner at lower pressure differentials and towards the runner at higher pressure differentials so as to restore the sealing function along the seal gap.
Future high-speed turbine engines, including those exceeding Mach 4, will require seals capable of withstanding temperatures in excess of 1500° Fahrenheit and rotational speeds in excess of 1500 feet-per-second, while minimizing leakage at critical locations in order to properly manage secondary flow. The ability to control secondary flow systems directly impacts component efficiencies and performance, component temperatures and thermal gradients, and component clearances over the operational range of a gas turbine engine. The control of such systems will become even more critical as the cooling temperatures within cooled cooling air (CCA) systems are reduced to further improve the performance of advanced engines. Unfortunately, the hydrostatic seal described in U.S. Pat. No. 6,145,840 does not adequately address the sealing challenges associated with advanced engines.
As is readily apparent from the discussions above, the related arts do not include an air-riding seal capable of providing the film stiffness necessary to prevent contact between a seal and seal runner at the speeds and temperatures of advanced engines.
Accordingly, what is required is a non-contact hydrostatic seal that provides a thin film which is sufficiently stiff to prevent contact between the seal and a rotating component under the operational conditions of advanced turbine engines.